This invention relates to a component of an iron, cobalt or nickel-base alloy having a protective arrangement to prevent aluminizing or chromizing during gas diffusion coating that includes a first layer serving as an interlayer and a second layer serving as a getter layer for reaction gases.
When producing protective coatings, such as aluminum or chrome diffusion coatings, by means of the powder packing method in accordance with U.S. Pat. No. 3,079,276 or U.S. Pat. No. 3,801,353 or by means of the vapor phase deposition method in accordance with U.S. Pat. No. 4,132,816 or German Patent Document DE 38 05 370, it has proved difficult to protect component sections from aluminizing or chromizing that should not be exposed to the coating atmosphere. In conventional protective arrangements the first layer consists of a fill of Al.sub.2 O.sub.3 particles and a getter layer consisting of a metal-metal oxide powder blend. Such powder fills are commercially available.
At coating temperatures above 900.degree. C. it is especially difficult to ensure high absorption capacity of the conventional protective arrangement for the elements to be deposited and for the aggressive halide-containing reaction gases without contaminating, etching, corroding, diffusion hardening, carbon- or oxygen embrittling or in any other manner altering the component area and surface to be protected. Also, at such elevated temperatures, components of the protective arrangement may chemically react to alter the protected component surface, which again gives rise to problems. Additionally, particles of the first layer of the protective arrangement may bond with the component by sintering or diffusion.
Conventional protective arrangements, such as shielding jeopardized component areas by powder fills or arranging jeopardized component areas in powder fills outside the reaction chamber have so far proved practicable at negligible scrap rates only for gas diffusion coating below 900.degree. C. In conventional protective arrangements, scrap is produced as a result of creep coating induced by the highly active, greatly reactive and scattering coating atmosphere at elevated temperatures. Also, damage to the protected component surfaces several micrometers deep and varying with time and temperature has been noted.
The present invention aims to provide a generic protective arrangement that overcomes the noted disadvantages and ensures complete protection of the component also when gas diffusion coating at temperatures above 900.degree. C. The present invention also provides a simple low-cost method for manufacturing, depositing and removing the protective arrangement.
It is a particular object of the present invention to provide an arrangement in which the first layer is a slip cast layer composed of oxide ceramic particles carried in a low-carbon vehicle free from halide, while the second layer is a metal layer or a metallic slip cast layer, where the metal layer exhibits at least 50% by weight of the base metal fraction of the component and contains all major alloy constituents of the component.
This protective arrangement provides an advantage in that it ensures full protection for the component area to be protected from aluminizing and chromizing during vapor-phase deposition also at process temperatures above 900.degree. C. Suitable matching of the material composition of the gettering and sealing metal layer, which is entirely free of oxide ceramic constituents, consists at least 50% by weight of the component's base metal and contains all major alloy constituents of the component, provides a further advantage in that this metal layer exhibits neither diffusion effects nor chemical reactions with the component material to be protected in the surface zones of the masking protective arrangement, especially so when the metal layer is chemically identical with the component material. The metal layer in the inventive composition protects the material of the component from aluminizing or chromizing by absorbing the elements (Al or Cr) and metal halides presented in the vapor phase, while on account of its metallic composition it will not alter the surface zone of the protected component area. Another advantage is provided in that the component surface is not depleted of elements easily removed by diffusion, such as aluminum, titanium or chrome, considering that in accordance with this invention, these are presented in sufficient concentration by the metal layer.
In a preferred aspect of the present invention the protective arrangement will not cover the component in its entirety, but only selectively, so that the surface areas of the component to be aluminized or chromized are fully exposed to the reaction gases.
To still improve on the action of the protective arrangement, the arrangement preferably exhibits several layers reacting with the coating atmosphere. These layers have metal particles of varying compositions, where the metal layer that in terms of composition corresponds most to the component alloy is preferably arranged closest to the component surface. The metal layer which on account of its composition or porosity has a pronounced gettering effect on the reaction gases is used as the central layer, while the outer layer is a maximally gastight layer, preferably metal foil or metal sheet masking.
In a further preferred aspect of the present invention the metal layer is arranged as a conformal metal foil or conformal metal sheet on the interlayer. Conformation to contour is here achieved either by pressing the metal foil onto the interlayer or by subsequently filling the space between the metal sheet and the first layer with metal slip. This aspect of the present invention provides an advantage in that the metal layer can be given an extremely effective gas sealing effect relative to the reaction space.
An additional contour-hugging masking sheet can be used to seal the metal layer preferably relative to the reaction space. A masking sheet of this description adds to the soundness of the protective arrangement and simultaneously facilitates the manipulation of the components when preparing for and performing the gas diffusion process. The masking sheet is here designed to split preferably along the molding base. In this manner the sheet is advantageously removable in halves from the component following the gas diffusion coating.
In a particularly preferred aspect of the present invention the base metal fraction in the metal layer does not deviate by more than 20% by weight from the base metal fraction of the component. This restriction advantageously ensures that in terms of material composition, the metal layer and the component are more precisely matched to one another and that diffusion voids or such other lattice defects as may otherwise arise in the presence of differing work functions in the rim areas between the metal layer and the component are avoided.
The fraction of chrome content in the metal layer should deviate from a potential chrome fraction of the component by not more than 20% by weight to avoid defects in the component surface. Similarly, the fraction of a Mo, Co or W content in the metal layer should vary from a potential Mo, Co or W content of the component by not more than 10% by weight, while the fraction of an Al or Ti content in the metal layer should vary from a potential Al or Ti content of the component by not more than 5% by weight.
For clean separation of the protective arrangement from the component following vapor-phase diffusion, the interlayer preferably consists of Al.sub.2 O.sub.3. The thickness of this layer preferably runs between 0.3 mm and 2 mm.
A method for manufacturing a component of an iron, cobalt or nickel-base alloy with a protective arrangement to prevent aluminizing or chromizing during gas diffusion coating with a first layer as an interlayer and a second layer as a getter layer for reaction gases uses the following process steps:
a) Deposit a first slip cast layer composed of oxide ceramic particles carried in a low-carbon vehicle free from halide; PA1 b) Deposit a second layer of metal or metallic slip, where the base metal of the metal layer contains at least 50% by weight of the base metal fraction of the component and the metal layer exhibits all major alloy constituents of the component.
This method provides an advantage in that it is suitable for bulk production, especially so as the slip casting process is a readily manageable technique giving reproducible coating results and thicknesses. For this purpose, a slip casting layer of preferably oxide ceramic particles is deposited by means of a vehicle consisting of polyvinyl alcohol or polyvinyl acetate and water. The metal layer as a gettering layer can be deposited to hug the contour also by plasma spraying, electroplating, painting with inorganic metal paints, vacuum vapor deposition or foil plating.
In a preferred aspect of the inventive process the component is partially fitted into a closely contoured outer sheet which does not react chemically with the component material and which in composition corresponds to the metal layer and then the space in between is fully or partially filled with slip for the first layer, or when the first layer has previously been deposited, with the material for the second layer, and the slip is then dried. This practice has shown that the slip between the closely contoured outer sheet and the component, if deposited simply as a seam like a weld bead, will provide perfect protection. With this process variant a compact assembly of protective coating layers of the protective arrangement and a closely contoured outer sheet for masking can be economically manufactured. The slip cast layer can here consist of oxide ceramic particles (first layer or interlayer) and be deposited preferably by means of a vehicle consisting of polyvinyl alcohol and water.
This protective arrangement is preferably used for protecting components or component areas from aluminizing or chromizing during gas diffusion coating at temperatures above 900.degree. C. and a high surface layer activity greater than 20% by weight Al or Cr for the layer deposited by gas diffusion. It has here been seen that practically perfect identity of the gettering metal layer with the component material alone will reproducibly ensure that the surface of the protected component area remains free from damage.